This invention relates to a beam direct converter used with a neutral beam injector (hereinafter abbreviated as NBI) for heating a plasma.
FIG. 1 is an exploded oblique view of the NBI fitted with a beam direct converter. FIG. 2 indicates the potential distribution in the NBI elements, showing the lateral sectional view of the NBI. A gas to be ionized, for example, hydrogen gas or deuterium gas is fed into a discharge chamber 10. Voltage is applied between the thermionic cathode 10a of the discharge chamber 10 and anode wall 10b (which is set at, for example, 200 KV as shown in FIG. 2), giving rise to the discharge of a gas and the growth of a plasma. Ions such as H.sup.+ or D.sup.+ are drawn off from the plasma by means of a grid electrode 12, and conducted at an accelerated speed to a neutralization cell 14 (which is, for example, grounded as shown in FIG. 2). The neutralization cell 14 is filled with, for example, a gas of hydrogen or deuterium having a relatively high pressure. A high energy ion beam accelerated by a grid electrode is subjected to a charge exchange reaction between gas molecules and high energy ions in the neutralization cell 14, and turned into an accelerated neutral beam. This accelerated neutral beam is injected into a plasma 28 by being guided through a drift tube 26. A vacuum vessel 22 (FIG. 1) in which the neutralization cell 14 is set is evacuated by a cryo pump 24.
That portion of an ion beam which was not neutralized by the neutralization cell 14 is also drawn off therefrom. The unneutralized ions are deviated from the flow of a neutral beam due to a self electric field being created by said unneutralized ions and strike against the walls of the vacuum vessel 22 and drift tube 26. These walls are damaged by the energy released from the unneutralized ions. As a plasma 28 is applied in an increasing size, a neutral beam must have a higher energy. However, a neutralizing efficiency will be reduced if a neutral beam is made to have a high energy. Where, with a beam of deuterium, an accelerated voltage stands at 50 KV, neutralization advances beyond 80%. In contrast where an accelerated voltage indicates 200 KV, neutralization is achieved only to an extent to 20%. Therefore, the energy loss of the unneutralized ions permeating the neutralization cell 14 and the damage to the walls of the vessel 22 and drift tube 26 raise greater problems.
For resolution of such problems, a beam direct converter for recovering ion energy in the form of a current (which comprises electron suppressors 16, 20 and a collector 18 provided therebetween) has hitherto been interposed between the neutralizing cell 14 and drift tube 26. As seen from FIG. 2, the voltage of the collector 18 and electron suppressors 16, 20 are respectively set at levels of 180 KV, -60 KV and -20 KV. An accelerated ion beam is decelerated by an electric field produced between the collector 18 and electron suppressor 16. The decelerated ion beam is taken into the collector 18, where the energy of said beam is recovered in the form of current. On the other hand, the electron suppressors 16, 20 held at a negative potential collectively build up an electrostatic potential barrier against electrons generated in other members (for example, the neutralizing cell etc.) than the beam direct converter, thereby preventing electrons from being carried into the collector 18 and consequently avoiding the loss of current recovered by the collector 18.
With the conventional beam direct converter, however, gases flowing out of the neutralization cell 14 and gasified deuterium ions prevail near the electron suppressors 16, 20 and collector 18. Collision between an ion beam and the gas molecules lead to the growth of charged particles in the beam direct converter. Ions generated in the beam direct converter collide with the electron suppressors 16, 20, leading to not only current loss, but also the generation of heat and the release of secondary electrons. Entry of said released secondary electrons into the collector 18 results in the loss of recovered current and a reduction in the recovery efficiency of the direct beam converter. When a D.sup.+ ion beam accelerated to 200 KeV is recovered by a residual energy of 20 KeV, it is assumed that the electron suppressor 16 is impressed with a voltage of -60 KV, an electric field created in the recovery region has a strength of 7 KV/cm, and a pressure of 1.times.10.sup.-4 Torr. Then about 2.5% of an incident ion power brought into the beam direct converter is lost by the thermal load of the electron suppressor 16. This loss indeed can not be overlooked, but falls within an allowable range, because said loss does not constitute an important portion of the loss of incoming ion power occurring in the whole of the direct converter. In this connection, it is to be noted that electrons are released over incoming ions. When the electron suppressor 16 is prepared from, for example, molybdenum, the secondary electron emission coefficient is about 3/ion. Therefore, ion power loss caused by released secondary electrons accounts for as much as about 6.3% of incident ion power delivered to the beam direct converter. This loss is extremely large, when it is considered that the residual incident ion power which is recovered in the form of current accounts for 10% of the incident ion power. The generation of charged particles in the beam direct convertor can indeed be suppressed by a reduction in the pressure prevailing in the vacuum vessel 22. The attainment of this object, however, is accompanied with the drawback that the cryo pump 24 must have increased capacity.
Ions produced outside of the beam direct converter or primary ions reflected from the surface of the collector 18 are also brought into the electron suppressor 16. As a result, this electron suppressor 16 releases heat, and emits secondary electrons. Said secondary electrons also lead to the loss of recovered ion power.